Heretofore, conventional spark plugs used, for example, in an internal combustion engine, a configuration in which a noble metal tip is provided on a distal end portion of a ground electrode via an intermediate member (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-163923). (See also Japanese Patent Application Laid-Open (kokai) No. 2009-158408, Japanese Patent No. 4073636 and Japanese Patent Application Laid-Open (kokai) No. 2008-214734). In the case where a noble metal tip is provided on the ground electrode, when the ground electrode is located so as to be away from a discharge gap, flame-quenching effect, which the ground electrode deprives energy of a flame kernel generated through discharge, can be prevented. Therefore, in the case where the noble metal tip is bonded to the ground electrode, when an intermediate member is provided, the amount of the noble metal employed can be reduced, and a sufficient distance can be secured between the ground electrode and the discharge gap.
Although the aforementioned intermediate member is formed of, for example, a nickel alloy, which is generally known to have very high corrosion resistance, when the intermediate member is employed for a long period of time, the member may corrode to an unacceptable extent. Particularly, in the case of an intermediate member, which is provided in the vicinity of a discharge gap of a spark plug, corrosion due to cooling-heating cycles is likely to occur. Thus, demand has arisen for further improvement of the durability of a structure in which a noble metal tip is attached to a ground electrode via an intermediate member. Since attachment of a noble metal tip via an intermediate member may be applied not only to a ground electrode but also to a center electrode, the aforementioned problems may occur not only in the ground electrode but also in the center electrode.
The present invention has been accomplished for solving the aforementioned problems, and an object of the present invention is to improve the durability of a structure in which a noble metal tip is attached to a ground electrode or a center electrode via an intermediate member.